Cardinal Francis George of Chicago addresses the U.S. Conference of Catholic Bishops in Baltimore on Nov. 15, 2010. / Steve Ruark, AP

by Melanie Eversley, USA TODAY

by Melanie Eversley, USA TODAY

Chicago's Cardinal Francis George issued a pastoral letter declaring his opposition to a proposed state bill that would legalize gay marriage, the Chicago Tribune and other media organizations report.

In a letter to priests, George wrote, "Civil laws that establish 'same sex marriage' create a legal fiction." He further wrote, "The State has no power to create something that nature itself tells us is impossible."

George's letter is in response to a proposed Illinois bill dubbed the Religious Freedom and Marriage Fairness Act that could be introduced by two Illinois state legislators this week, the Tribune reports.

The two Chicago Democrats who could introduce the bill - state Rep. Greg Harris and state Sen. Heather Steans - say public support for equal rights for same-sex partners is growing rapidly, NBC Chicago reports.

Roman Catholic leaders who have expressed opposition to gay marriage have cited natural law, which says one of the purposes of sexual relations is to procreate, according to the Tribune.

In the letter, George urged parishioners to contact state legislators and oppose the proposed legislation, the Tribune reports. But he reminded parishioners they should not alienate members of the gay community, especially family members, according to the news organization.

"Good pastoral practice encourages families to accept all their children and not break relationships with them," George wrote.